In this proposal we seek to determine the three-dimensional structures of the enzyme chloroperoxidase. The three-dimensional X-ray structure of chloroperoxidase will provide a framework for critically testing proposed enzyme mechanisms and in so doing provide a model for biological halogenation which may be applicable, for example, to the iodination of tyrosine in the biosynthesis of thyroxin. The similarity of chloroperoxidase to horseradish peroxidase, cytochrome C proxidase, thyroid peroxidase and cytochrome P-450 may allow structural and mechanistic inferences to be made in these other enzyme systems. The cytochrome P-450 analogy is of particular interest in light of its involvement in the conversion of potentially carcinogenic benz(a)pyrenes to active carcinogens. Since chloroperoxidase is a glycoprotein, the three-dimensional structure of chloroperoxidase will also provide the data necessary for elucidation of the interaction between protein and bound carbohydrate which might be an aid in understanding both how glycoproteins are synthesized and how protein and carbohydrate might interact in both extracellular and cell surface glycoproteins.